The development of the use of gelled hydrocarbons as fracturing fluids is reviewed by Weldon M. Harms in a chapter entitled "Application of Chemistry in Oil and Gas Well Fracturing", at pages 59-60of the book "Oil-Field Chemistry (ACS Symposium #396-1988)" published by the American Chemical Society in 1989. The basic technique of formation fracturing involves the injection of a fracturing fluid down the well bore, which is usually cemented in place and at least 0.3 mile long, and then through horizontal holes in the steel pipe, or casing, of the well, to obtain access to the subterranean formation. The fracturing fluid is under high pressure and must be able to survive the severe shear forces caused when flow is forced through the casing perforations of perhaps 1/4 to 1/2 inch in diameter, as well as the shear forces encountered at the leading edge of the fracture. Whatever chemical additives are used to influence viscosity, induce gel formation, stabilize against resident chemicals, pH or temperature conditions in the formation, inhibit scale formation or corrosion, or inhibit paraffin deposition, for example, must also be able to withstand the shear forces and other inhospitable conditions of use. Most commonly available liquids typically are viscosified before they are particularly effective in carrying the large quantities of proppants widely used in the fracturing process.
When hydrocarbons are used in the fracturing process, they are commonly treated to increase their viscosity. As reviewed by Harms, an early viscosifying agent was napalm, an aluminum soap of fatty acids. Aluminum salts of orthophosphate esters were introduced in the late 1960's, followed by the suggestion of the use of Fe.sub.3 O.sub.4 for combination with the orthophosphate esters, in Monroe U.S. Pat. No. 3,505,374. While many other combinations of metals and other materials have been suggested as viscosifying agents, aluminum crosslinked orthophosphate esters are still, according to Harms, the most widely used.
The aluminum compounds present problems, however, particularly where any significant amount of water is present. They generally will not satisfactorily perform the desired crosslinking function in the presence of more than about 1200 ppm of water, nor where the pH is outside a relatively narrow range. Moreover, an inadvertent excess of aluminum compound treatment is detrimental to the desired performance because the aluminum compound itself adversely affects the pH. The iron provided by ferric salts as in the present invention, on the contrary, permits operation in wider pH ranges.
In describing a gel which can be used as a pig in a pipeline, Jaggard et al in U.S. Pat. No. 4,003,393 recite the possibility of iron as one of a number of metals to combine with a class of aliphatic substituted orthophosphoric esters. No other qualifiers are used to describe the iron, however.
In U.S. Pat. No.4,153,649, Griffin proposes reacting a pentavalent phosphorous compound with a class of hydroxy ethers before employing the metal salt. Among the metal salts he uses is ferric nitrate, but he further requires a "separate source of base" to be used with the hydroxy ether modified phosphates, as spelled out in column 4, lines 55-58 and column 11, lines 37-68.
Monroe, in U.S. Pat. No. 3,505,374, uses a gelling agent for hydrocarbons characterized as a ferroso-ferric salt of an alkyl oleyl diester of orthophosphoric mono acid. The iron compound is further described as magnetite, or Fe.sub.3 O.sub.4. He suggests this combination for fracturing subterranean oil-bearing formations, but says none of the "other oxidized forms of iron including ferrous and ferric oxides and hydroxides, chlorides, sulfates and nitrates" (col 3, lines 2-4) yielded a gel as obtained with the magnetite.
Burnham, in U.S. Pat. No. 4,200,540, describes a large class of phosphates and phosphate esters which he mixes with aluminum salts, aluminates and aluminum metal. He chooses combinations of the materials as a function of various down-hole temperatures. No mention is made of iron salts; the reference is cited mainly for its comprehensive description of the phosphates deemed to be useful. See also Burnham's U.S. Pat. No. 4,316,810.